Older women who undergo in vitro fertilization are often encouraged to shell out for pre-implantation genetic screening in an effort to determine the best embryos to implant. But there's increasing evidence that the screening does more harm than good.

Gautam Naik writes in the Wall Street Journal that, despite a 2007 guidance from the American Society for Reproductive Medicine, there is zero evidence that PGS increases the rate of live births among older women undergoing IVF procedures, and doctors have continued to perform (and encourage women to have) it. Why? Naik has a potential answer.

Backed by limited positive data from a decade ago, some U.S. fertility clinics have aggressively marketed the PGS technique to older women. It isn't cheap. Embryo screening can add $2,000 or more to the typical cost of roughly $10,000 per treatment cycle of in-vitro fertilization. While some insurance companies cover IVF treatment, very few pay for PGS, which insurers consider experimental.

In other words, PGS is the new undercoating.

While doctors who use and market PGS in their practices despite the ASRM's guidance defend its use and effectiveness, rigorous studies showing otherwise have been available since 2004.

A randomized trial in 2004, in which a Belgian research team studied nearly 300 fertilization procedures, was among the first to indicate that embryo screening may not improve the live birth rate.

A recent Swedish trial, which included more than 100 women over the age of 38, suggests that embryo screening may actually hurt a woman's chances of having a baby. The study found that the pregnancy rate in a PGS group was 8.9% compared with 24.5% in a control group that didn't use PGS. The study, published in the journal Human Reproduction in December, was stopped midway after the data showed the sharply lower pregnancy rate.

Most randomized studies (and sometimes even the doctors don't know which is the control and which is the experimental group) are only stopped if the evidence shows convincingly that they are doing more harm than good.

Scientists have a potential hypothesis for why the post-PGS pregnancy rate was nearly 3 times lower than the non-PGS rate: PGS is often performed on a 6-cell embryo, likely to mitigate the political hot potato of destroying the embryos that are found to be malformed. This, however, might represent a bigger problem than doctors initially envisaged.

Doctors postulated that in performing PGS, the process of removing a single cell could damage other cells in the embryo. "The probability of doing harm with PGS turned out to be higher," says Thorir Hardarson, biologist at Carlanderska Hospital Fertility Center in Gothenberg, Sweden, who led the trial.

Doctors looking at embryos from otherwise healthy fertile couples have a further explanation, after discovering that the rate of abnormality in their PGS screenings was very despite the theoretical probability being very low.

The answer appears to be that the single cell extracted from each embryo could be genetically slightly different from the others, and therefore not representative of the overall embryo, a condition known as mosaicism. In fact, removing an error-free cell from an embryo might lower the chances that that embryo will grow into a healthy baby, because the remaining cells might not be as viable, Dr. Vermeesch says

Either way, women being pressured into or sold on PGS as a way to improve their chances at conceiving through IVF are paying money for something that, at best, has little effect on their chances of conception and, at worse, might actually be harming it.